Nitrogen narcosis is a change in consciousness, neuromuscular function, and behavior brought on by breathing compressed inert gasses. It has also been called depth intoxication, “narks,” and rapture of the deep. Traditionally the gas involved in narcosis is nitrogen, and it is associated with dysfunction when breathed by scuba divers from their tanks containing compressed air. Other inert gasses associated with narcosis include neon, argon, krypton, and xenon, with the latter having an anesthetic effect even at sea level. Interestingly, helium does not cause inert gas narcosis and therefore, is used in deep diving as heliox (helium and oxygen mixture).
In underwater diving, narcosis (nitrogen narcosis, inert gas narcosis, raptures of the deep, Martini effect) is a reversible change in consciousness that occurs while a person is diving at deep depths. Certain gases at high-pressure cause an anesthetic effect that alters a diver's consciousness.
Due to variability in reporting of information, publication of diving-related injury statistics is inconsistent. The Divers' Alert Network (DAN) acts as a medical information and referral service for diving-related injuries. According to DAN, fewer than 1% of divers experience decompression sickness (DCS).
Breathing compressed air while at atmospheric pressures greater than 1 ATM increases the partial pressures of nitrogen and oxygen, in the blood. The nitrogen atoms inhaled in the compressed air while at pressure remain chemically unchanged in the blood, leading to the belief that there is a physical component to the involvement of nitrogen in causing narcosis. The effect of nitrogen on the body takes place in the central nervous system (CNS), but the exact site and mechanism are still debated. The lipid solubility hypothesis by Meyer and Overton noted that there is a correlation between the solubility of an anesthetic in lipid and its narcotic power. They also stated that "all gaseous and volatile substances induce narcosis if they penetrate cell lipids in a definite molar concentration which is characteristic for each type of cell." This theory was expanded by applying the "critical volume" concept which states that narcosis occurs when the inert gas or anesthetic changes a lipid portion of the cell. This is often thought to be the cell membrane, causing that portion of the cell to swell to a certain volume, impairing its function for that specific cell type.
The effects of nitrogen narcosis are highly variable among divers with all divers being significantly impaired while breathing air at 60 to 70 meters, whereas some divers are affected at 30 meters. The effects are not progressive with time while depth is maintained, but symptoms progress and new symptoms develop as a diver descends deeper to greater pressures. The narcotic symptoms observed are quickly reversible upon ascent.
The symptoms seen in nitrogen narcosis begin first with effects of the higher function such as judgment, reasoning, short-term memory, and concentration. The diver may also experience a euphoric or stimulating feeling initially similar to mild alcohol intoxication. Further increases in the partial pressure of nitrogen in the blood from descending deeper lend to impairments in manual dexterity and further mental decline including idea fixation, hallucinations, and finally stupor and coma. Death can result from unconsciousness associated with severe narcosis or from severely impaired judgment leading to an accident of some form during the dive. Other factors have been linked to increased risk of nitrogen narcosis during dives while breathing compressed air and they include alcohol, fatigue, anxiety, and hypothermia. The concentration of carbon dioxide in the blood is thought to have an additive, rather than synergistic effect to nitrogen narcosis.
Measuring the effects of nitrogen narcosis have been studied and have been broken down into looking at cognition and behavior as well as neurophysiological changes that occur during narcosis. One study on simple arithmetic and reaction time showed the qualitative progressive decline in the participant’s ability to complete the test’s tasks with increasing pressures while breathing compressed air. Another study designed to see the difference between nitrogen narcosis in a laboratory setting versus in an open water setting. It reported that "divers were tested at 3 and 30 meters at a shore base and in the open sea. Intellectual functions, as assessed by memory test, sentence comprehension, and simple arithmetic, showed evidence of narcosis in both 30-meter dives but the decrement was greater in the ocean dives. This may be due to the greater psychological stress in the open sea," hinting at a greater impairment from narcosis when the diver is also experiencing anxiety from other factors outside of the narcosis.
Neurophysiological testing has been done using an electroencephalogram to measure brain wave activity. Studies have found decreased voltages in the basal rhythm of the brain and the appearance of low voltage theta waves. Cortical evoked potentials were also studied. These evoked potentials are differences on electroencephalogram readings that show a response in the brain to sensory stimuli, such as visual images. US Navy divers were studied using these visual evoked responses and showed that reliable and significant differences in visual evoked responses were apparent in the divers when they were breathing compressed air at depth, and these differences were not apparent when breathing compressed helium-oxygen mixtures.
Nitrogen narcosis can be prevented in different ways. Limiting the depth of a dive is one of the least invasive. It is agreed upon that the maximum depth limit for a diver to use compressed air is 30 to 50 meters. Beyond this, a gas mixture other than air is suggested for use to prevent nitrogen narcosis. For dives greater than 50 meters the diving community commonly substitutes helium or helium-nitrogen as the diluent gasses for oxygen. Helium has not been shown to have an anesthetic or narcotic effect on divers at depth breathing compressed gases. Helium imposes a greater decompression burden upon divers than does nitrogen.
Treatment of narcosis involves early recognition of symptoms by the diver or the diver’s partner while at depth. Symptoms resolve completely within minutes upon ascent of the diver. If the symptoms persist, the dive should be aborted.
Inert gas narcosis completely resolves upon ascent. It poses no problem in the long term and does not lead to chronic issues or predispose to increased (or decreased) susceptibility to recurrent exposures. However, inert gas narcosis will recur on exposure to pressure, it is unpredictable in severity and can lead to fatalities while diving, due to cognitive impairment in the marine environment.
Although helium does not cause inert gas narcosis, it can cause at deep depths. This is an unrelated phenomenon which is interestingly treated by adding nitrogen back into the breathing mixture.